The invention relates generally to lithograph offset printing presses and, more particularly, to an improved shell for use in place of a standard rubber blanket when applying a perf, score or slit to a substrate in an offset printing press.
As shown in FIG. 1, a typical offset press printing group includes a plate cylinder 20, an impression cylinder 22 and a blanket cylinder 24. The blanket cylinder 24 is covered with a rubber blanket 26 and contacts the impression cylinder 22. A printing plate 28 featuring text or images is attached to the plate cylinder 20. Ink is supplied to the plate cylinder 20 so that the text or images of the printing plate are transferred onto the blanket cylinder 24 and ultimately onto a substrate, such as paper 32.
As illustrated in FIG. 1, perforating material 34 may be placed about the impression cylinder 22 so that the printing group applies perforations to the substrate 32 instead of printed text or images. Slitting or scoring material may be substituted for, or used in addition to, perforating material 34. As illustrated in FIG. 2, typical perforating material includes spaced teeth 40 with pointed or curved tips 42 that dig into the rubber blanket 26 of the blanket cylinder 24 (FIG. 1).
The perforating arrangement of FIGS. 1 and 2 have two significant disadvantages. First, the process damages the rubber blanket of the blanket cylinder over time so that reuse of the blanket is prevented. Second, a quality defect called a "shoulder" is produced on the substrate due to the flexibility of the rubber blanket. A shoulder, illustrated at 46 in FIG. 3, is a raised area on the substrate 32 that runs parallel to the perforation 48 (or score). In some instances, the shoulders may become severe enough that sheets of the substrate cannot be stacked for ease of subsequent processing.
In an attempt to overcome the above problems, shells or foils that are substituted for the rubber blanket of the blanket cylinder during perforating, slitting or scoring operations have been introduced. An example of such an arrangement is presented in U.S. Pat. No. 5,623,874 to Ruprecht. While the prior art arrangements have been effective, their performances have been less than optimal. In addition, installation of the prior art devices could be facilitated.
Accordingly, it as object of the present invention to provide a perf/score shell for presses that enables perforations, slits and scores to be formed in a substrate without creating shoulder defects.
It is a further object of the present invention to provide a perf/score shell for presses that may be used repeatedly.
It is a further object of the present invention to provide a perf/score shell for presses that enables perforations, slits and scores to be formed in a substrate going around the cylinders of the presses and across the cylinders of the presses with equal quality.
It is still a further object of the invention to provide a perf/score shell for presses that is easy to install using existing cylinder lock-ups.
It is still a further object of the invention to provide a perf/score shell that may be used on sheet-fed offset presses and the perf units of web offset presses.